These teachings relate generally to detectors of electromagnetic radiation and, more specifically, relate to focal plane arrays (FPA) of infrared (IR) radiation detectors, preferably microbolometer IR detectors, that are not cryogenically cooled (i.e., that are uncooled) during operation.
Thermal stability is an important consideration in the operation of uncooled FPA, such as FPAs that employ silicon-based microbolometer detection elements. The silicon microbolometer is inherently very sensitive to changes in temperature, which make its use as a detector of thermal energy particularly attractive. In an ideal case, all thermal inputs to the microbolometer array, with the exception of the scene being viewed, are identical. Thus, if the FPA were viewing a uniform black body, all of the microbolometers would be at exactly the same temperature and would have identical outputs. However, in practice it is impossible to achieve a uniformity of temperature distribution across the microbolometer array, resulting in the generation of a spatial non-uniformity thermal noise component and offset in the output signals of the FPA.
It can be shown that the thermal variations within a detector assembly containing the FPA and an associated readout integrated circuit (ROIC) can be in a typical range of about 1 K to about 2 K, for a given heat load and thermoelectric cooler operational temperature (e.g., 300 K). It can be further demonstrated that the actual thermal variation can be influenced by a number of factors, including the thermal conductivity of an adhesive bondline between the detector assembly and other components, such as a motherboard, as well as any variation in temperature along the thermal electric cooler, variations in the quality of the attachment of a vacuum package that contains the detector assembly to its mounting surface, as well as the magnitude of a thermal gradient between the detector assembly operating temperature and the environmental temperature, such as may occur when operating in a high ambient temperature environment.
It is known in the art to use the thermal electric or other type of cooler to cool the microbolometer array to an (ideally) uniform temperature. It is also known to use a separate resistor/heater element in an attempt to equilibrate the temperature across the FPA. The use of the separate thermal electric cooler or the resistor/heater element, however, increases the cost, complexity and volume requirements of the overall detector assembly.
It is also known in the art to employ high thermal conductivity materials, such as copper and aluminum nitride, in an attempt to smooth out thermal spatial variations across the FPA. However, this approach can also suffer from the problems inherent in the thermal electric cooler or resistor/heater approaches and, furthermore, makes no provision for the possibility of the thermal spatial variations changing during operation.
In U.S. Pat. No. 5,756,999, xe2x80x9cMethods and Circuitry for Correcting Temperature-Induced Errors in Microbolometer Focal Plane Arrayxe2x80x9d, W. J. Parrish and J. T. Woolaway describe various techniques for the correction of temperature-induced non-uniformities in the response characteristics of microbolometers in an IR-FPA. Referring to FIGS. 5A and 5B, in this prior art approach a thermally-shorted microbolometer is employed to sense the substrate temperature, and circuitry is used to drive an on-ROIC resistor heater to heat the ROIC substrate to a constant temperature. A voltage source (VS) is used to set the desired ROIC substrate temperature.
As can be appreciated, this approach does not adequately address the problem of localized temperature differences that typically exist across the ROIC. For example, and referring to FIG. 5B, it can be seen that those microbolometers that are nearest to the resistor heater will most likely be maintained at a temperature that differs from the microbolometers that are furthest from the resistor heater. In addition, the significant spatial separation between the temperature sensor (i.e., the thermally-shorted microbolometer) and the resistor heater element may result in a less than optimum tracking of the temperature.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
A focal plane array (FPA) of infrared (IR) radiation detectors, such as an array of microbolometers, includes an active area comprised of a plurality of IR radiation detectors, a readout integrated circuit (ROIC) that is mechanically and electrically coupled to the active area and, disposed on the ROIC, a plurality of heater elements that are located and operated so as to provide a substantially uniform thermal distribution across at least the active area. The FPA further includes a plurality of temperature sensors, individual ones of which are spatially associated with one of the heater elements for sensing the temperature in the vicinity of the associated heater element for providing closed loop operation of the associated heater element. In one embodiment pairs of the heater elements and associated temperature sensors are distributed in a substantially uniform manner across at least a top or a bottom surface of the ROIC, while in another embodiment pairs of the heater elements and associated temperature sensors, or only the heater elements, are distributed in accordance with a predetermined thermal profile of the FPA. The plurality of heater elements may each be comprised of a silicon resistance, and the plurality of temperature sensors may each be each comprised of a silicon temperature sensor.
These teachings thus provide in one aspect a FPA of radiation detectors that includes an active area containing an array of microbolometers; a ROIC that is mechanically and electrically coupled to the active area and a plurality of temperature sensors and heater elements operating in a closed loop manner for substantially minimizing a temperature gradient across at least the active area.